IOSR Journal of Applied Geology and Geophysics (IOSR-JAGG) e-ISSN: 2321–0990, p-ISSN: 2321–0982.Volume 4, Issue 5 Ver. I (Sep. - Oct. 2016), PP 52-63 www.iosrjournals.org DOI: 10.9790/0990-0405015263 www.iosrjournals.org 52 | Page Organic Geochemical Evaluation and Depositional Environment of Oil Sand from Mbano, SE Nigeria M.U. Uzoegbu* 1 , N.R. Nwachukwu 2 and O. Wosu 2 1 Michael Okpara University of Agriculture, Umudike, Abia State, Nigeria 2 University of Port Harcourt, Port Harcourt, Rivers State, Nigeria Abstract: In this study, organic geochemical characteristics and depositional environment of the Cretaceous- Tertiary aged oil sand deposits in Mbano Southeast Nigeria have been examined. Oil sands in all the studied areas are typically characterized by high hydrogen index and low oxygen index values. The organic richness of the oil sand, the quality, source and thermal maturity of organic matter discovered at Umuezeala Nsu (MBA) and Umualumaku (MBB) in Mbano were determined on 13 oil sand samples. The results generally suggest relatively high Total organic matter content (TOC) for MBA and MBB oil sand samples ranging from 17.07 to 61.31wt% with a mean of 44.43 wt% for MBA and from 29.55 to 52.94 wt% with a mean of 36.05 wt% for MBB respectively. This serves as a link in determining the quality of source rocks. The free hydrocarbon (S1) versus TOC plot was used to differentiate between allochthonous and autochthonous hydrocarbon. Tmax ranges from 412-431 o C with mean value of 421.8 o C. Hydrogen index (HI) versus Tmax plot confirms that a significant amount of the organic matter is of lacustrine with a mixed marine and terrestrial origin. The HI of MBA and MBB ranges from 771 to 869 mgHCg -1 TOC and 689 to 89 0 mgHCg -1 TOC respectively indicating the presence of type I kerogen. The Tmax versus Production index (PI) shows that MBA and MBB organic matter as immature but the Production index (PI) ranges from 0.16 and 0.29 indicating that the organic matters in MBA and MBB have reached the production stage. The GC-MS results derived by CPI varies from 0.01 to 3.80 in MBA and 0.14 to 0.90 in MBB showing that MBA attended higher level of maturation as compared to MBB. Furthermore, indicated that MBA organic matter was deposited in a deeper environment than MBB or closer to heat source. Pr/Ph ratio (0.44-2.33; 0.68-3.55), Long/Short ratio (0.17-1.28; 0.10-0.66), OEP (0.40-1.22; 0.31- 0.93), Pr/n-C 17 (0.10-2.10; 077-3.27), Ph/n-C 18 (0.04-3.50; 0.43-1.72) and C 31 /(C 31 +C 29 ) (1.00-3.60) revealed the organic matter depositional environment as dysoxic to marine environment. Keywords: Oil sand, TOC, Tmax, production Index, depositional environment, Anambra Basin, Nigeria. I. Introduction The studied area lies within latitudes 4°45'N and 7°15'N, and longitude 6°50'E and 7°25'E with an area of around 5,100 sq km (Figure 1) and is within the Anambra basin.Oil sand is a naturally occurring mixture of sand, clay or other minerals, water and bitumen, which is heavy and extremely viscous oil that must be treated before it can be used by refineries to produce usable fuels such as gasoline and diesel. Oil sand which is also referred to as tar sand (Bituminous sand) has a similar composition as the light crude. It is believed to have been formed from biodegradation and water washing of light crude due to lack of cap rock (Akinyemi et al., 2013). Bitumen is about 20% of the actual oil sands found in Nigeria while 76% is for mineral matter that includes clay and sand and 4% water (Akinyemi et al., 2013). The recovery process includes extraction and separation systems to remove the bitumen from the sand and water. The oil sand history started with the development of oil sand separation in the 1920s by Dr. Karl Clark. In 1936 Max Ball developed a way to produce diesel oil from oil sand (Nate, 2008). Nate (2008) also reported that the actual commercial production started in 1963 when the Sun Oil Company – later Suncor – started the construction of the first commercial oil sand production plant. The first barrel of commercial production by open pit mining was produced in 1967 (Syncrude, 2003). Various countries of the world have embraced the exploration of oil sands as an alternate source of energy. In the Anambra basin, Southeastern Nigeria, oil sand deposits has been discovered in Mbano, Imo State. This study aims to determine the organic matter richness, quality, type, source, maturity variation, the geochemical characterization and depositional environment of the organic matter from the oil sand deposits.
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IOSR Journal of Applied Geology and Geophysics (IOSR-JAGG)
These values of OEP for immature higher plant contributions are often >1.0 but approach 1.0 with
increasing maturity (Bray and Evans, 1961; Didyk et al., 1978). The CPI results gotten from MBA and MBB
could be said that majority of the terrestrial material is probably transported by fluvial processes and deposited
under marginal to marine environments (Bird et al., 1995; Obaje et al., 2004) which contribute to the marine
source of OM in the MBA and MBB samples.
Table 2: Showing n-Alkanes, Isoprenoids distribution and ratios from GC of oil sand samples from Mbano,
Anambra Basin.
The ratio C31/ C31 + C29 of these two n-alkanes (C31 and C29) are higher in the sample numbers of MBA
and MBB both in oil sand samples with values 3.60 and 1.40 (Table 2) respectively. The carbon preference
index (CPI) of the n-alkanes also varies with values ranging from 0.14-0.90in the oil sand samples from MBB-
0.1 and 0.01-3.80 in the oil samples from MBA-02 (Table 2; Figure 4). This indicates more maturation of OM in
MBB area than the MBA area or location MBB may be closer to higher heat source.
The CPI 25-33= 0.5*[(C25-C33)/ (C24-C32)] + [(C25- C33)/ (C26-C34)] n-alkanes can be derived from two
sources i.e. vascular plant wax and fossil fuel combustion products (Ali et al., 2015). An odd carbon preference
is characteristic of oils derived from source rocks deposited in non-marine environments. In contrast, the
predominance of an even numbered n-alkane preference is commonly observed in bitumen and oils derived
from carbonate or evaporate rocks. This is usually characterized with CPI values that are lower than 1. If the
total even and odd numbers of paraffin are equally abundant, the value of (CPI) will be equal to 1 as generally
observed in high maturity samples. This shows that the hydrocarbons are petrogenic in origin.
Predominance of vascular plants input to the environment usually demonstrates CPI values from1 to 3
(Ali et al., 2015). The average CPI value of 0.35 was obtained from MBB samples and 1.35 from samples of
MBA suggesting marginal maturity. Although this parameter is usually low in marine source rock that produce
mostly low molecular weight hydrocarbons (Cooles et al., 1986; Peters and Moldowan, 1993; Eseme et al.,
2002; 2006; Sengüler et al., 2008). CPI in the C25-C31 range of samples from the Anambra Basin is generally
high (> 1), indicating immaturity and reflecting the contribution of wax-derived n-alkanes which form bitumen
at the end of diagenesis (Hunt, 1996).
Pr/C17 values range from 0.10-2.10 in the MBA sample and values from 0.77-3.27 in MBB while
Ph/C18 is between 0.04-3.50 in the MBA and 0.43-1.72 from MBB (Table 2; Figure 4) indicating marginal to
marine depositional environment. The result of C31/(C31+C29) (1.00-3.60) also supported values obtained for
Pr/C17 and Ph/C18 depositional environment of the organic matter as dysoxic to marine environment (Pingchang
et al., 2013).
Acknowledgements
Gratitude is expressed to all the Staff in the Departments of Geology, Michael Okpara University of
Agriculture and University of Port Harcourt for their advice. Trican Geological Solutions, Alberta, Canada is
gratefully acknowledged for the analyses of these samples.
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